Fuel cell is divided into four kinds depending upon the kind of the electrolyte used therein, i.e. molten carbonate fuel cell (MCFC), solid oxide fuel cell (SOFC), phosphoric acid fuel cell (PAFC) and polymer electrolyte fuel cell (PEFC). Lately, development is under way for fuel cell using an enzyme or a microorganism as a catalyst, i.e. bio cell.
Single cell as a unit of polymer electrolyte fuel cell comprises a thin-sheet-shaped polymer electrolyte membrane and gas diffusion electrodes (each having a catalyst layer) laminated to each side of the polymer electrolyte membrane. Incidentally, the gas diffusion electrode having a catalyst layer is called membrane-electrode assembly (hereinafter may be referred also as “MEA”). The polymer electrolyte fuel cell has a stack structure in which a plurality of the above-mentioned single cells are laminated via separators. The polymer electrolyte membrane allows the selective permeation of hydrogen ion (proton). The above-mentioned catalyst layer is composed mainly of fine carbon particles supporting thereon a noble metal catalyst made of, for example, platinum. The above-mentioned gas diffusion electrode is required to have a gas-diffusing property of introducing a fuel gas and air into the catalyst layer and exhausting the formed gas and an excessive gas, a high conductivity of taking the generated electricity outside with no loss, and a durability to the strongly acidic atmosphere caused by the generated proton.
As the material for the gas diffusion electrode, there is used, in many cases, a carbon fiber sheet (e.g. carbon fiber cloth, carbon fiber felt or carbon fiber paper) because it is superior in mechanical properties, acid resistance and conductivity and is light.
For production of the carbon fiber sheet, the following methods are mentioned, for example. There is a method of producing a carbon fiber sheet by making a carbon fiber (e.g. filament yarn, staple yarn or cut fiber) into a sheet by weaving, sheeting or the like. Also, there is a method for producing a carbon fiber sheet by subjecting a flame-resistant fiber (a carbon fiber precursor) to sheeting and carbonizing the resulting sheet at a temperature of 1,000° C. or more (for example, Patent Literature 1). Further, there is a method for producing a carbon fiber sheet by mixing a carbon fiber and a binder for sheeting, subjecting the mixture to sheeting, impregnating the resulting sheet with a thermosetting resin (e.g. phenol), setting the impregnated resin, and then conducting carbonizing at a temperature of 1,000° C. or more (for example, Patent Literature 2).
The gas diffusion electrode is required to have a function of exhausting the water formed by electricity-generating reaction, to a separator. The reason is that, if the water stays in MEA, the feeding of fuel gas to catalyst layer is hindered (this phenomenon may be hereinafter called “flooding”). In order to promote the exhausting of the water formed by electricity-generating reaction to suppress the flooding, the carbon fiber sheet constituting the gas diffusion electrode is generally allowed to have hydrophobicity. For allowing the carbon fiber sheet have hydrophobicity, it is generally conducted to impregnate a conductive sheet (e.g. carbon fiber sheet) with a water-repellent material such as polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF) or the like and subjecting the resulting sheet to sintering at 200 to 500° C. (for example, Patent Literature 3).
The gas diffusion electrode is required to have a function of uniformly diffusing a fuel gas in the catalyst layer and a function of controlling the wetness inside MEA. When a fuel gas is uniformly diffused in the catalyst layer, the area of reaction between fuel gas and catalyst layer increases. Further, when the wetness inside MEA is controlled, the drying of polymer electrolyte membrane is suppressed and the electrical resistance of polymer electrolyte membrane decreases. As a result, a fuel cell constituted using this gas diffusion electrode can generate a high voltage.
In order to allow the gas diffusion electrode to have a function of uniformly diffusing a fuel gas, it is generally conducted to form a micro porous layer (MPL) on a carbon fiber sheet. This MPL is constituted by carbonaceous particles (e.g. carbon black) and a fluoroplastic and has pores of about several μm in average diameter. A carbon fiber sheet having a MPL, as compared with a carbon fiber sheet having no MPL, can diffuse a gas uniformly. Also, a carbon fiber sheet having a MPL can hold water and accordingly can control the wetness. The MPL is formed, for example, by spraying or knife-coating a slurry containing carbonaceous particles and a fluoroplastic at appropriate concentrations. The coating is conducted generally by coating on the surface of a carbon fiber sheet (for example, Patent Literature 4).
As described above, as the gas diffusion electrode for polymer electrolyte fuel cell, there is generally used a water-repellent conductive sheet produced by subjecting a carbon fiber sheet to a hydrophobicity treatment and then forming a MPL on the resulting sheet.
This water-repellent conductive sheet is in wide use as an electrode material not only for polymer electrolyte fuel cell but also for fuel cell (e.g. bio fuel cell or air zinc cell) required to have diffusibility of fuel gas or liquid fuel and water-exhausting property.
However, the water-repellent conductive sheet is produced via many steps as described previously and therefore is low in production efficiency. As a result, a fuel cell using the water-repellent conductive sheet as an electrode material is expensive. Various proposals were made for these problems.
In Patent Literature 5, there is described a method for producing an electrode material for fuel cell, which comprises immersing a base material made of a polyarylate non-woven fabric, in a slurry wherein a fluoroplastic and a carbon material (e.g. carbon black) are dispersed and then drying the resulting material.
In Patent Literature 6, there is described a method for producing an electrode material for fuel cell, which comprises adhering, to a base material made of a glass fiber non-woven fabric, an acrylic resin or a vinyl acetate resin, then coating thereon a conductive paste which is a mixture of PVDF or PTFE and carbon particles, with a solvent, and drying the resulting material.
The production methods described in Patent Literatures 5 and 6 are superior to conventional methods in production efficiency. However, in these methods, the conductivity of the polyarylate non-woven fabric or glass fiber non-woven fabric used as a base material is low and, therefore, a fuel cell using the electrode material has a high internal resistance. As a result, no high cell performance is obtained.
Also, various proposals were made for an electrode material for fuel cell, having conductivity and high productivity (for example, Patent Literature 7 and Patent Literature 8).
In Patent Literature 7, there is described an electrode material for fuel cell, wherein a metal mesh is coated with a noble metal to allow the metal mesh to have higher acid resistance. However, since the noble metal used for coating is expensive, the fuel cell obtained is expensive. A metal mesh not coated with any novel metal invites the deterioration of electrolyte membrane by the metal ion dissolving due to corrosion. Accordingly, the metal mesh is required to be subjected to an acid resistance treatment such as coating of noble metal.
In Patent Literature 8, there is described an electrode material for fuel cell, produced by coating, on a base material sheet [made of a woven or non-woven fabric composed of an inorganic fiber (e.g. glass fiber) or an organic fiber, or a metal mesh], a carbon fiber, carbon fine particles and a resin. This electrode material contains carbon fine particles between carbon fibers and therefore has high conductivity, as compared with the electrode materials described in Patent Literature 5 and Patent Literature 6. However, production of an electrode material of high conductivity makes it necessary to impregnate the inside of base sheet with sufficient amounts of carbon fiber and carbon fine particles. In order to impregnate the inside of base sheet with sufficient amounts of carbon fiber and carbon fine particles, a low-concentration resin solution (wherein a carbon fiber and carbon fine particles are dispersed) is coated on a base material sheet, followed by drying, and this operation is repeated until a desired impregnation amount is reached. However, this method repeats coating of resin solution and drying and therefore is low in production efficiency.
Various proposals were made for a water-repellent sheet produced by sheeting (i.e. papermaking). The sheet produced by sheeting is high in production efficiency. In Production Literature 9, there is described a method for producing a water-repellent sheet, which comprises dispersing an aromatic polyamide and fluoroplastic particles in water and subjecting the dispersion to sheeting. The sheet obtained by this method has no conductivity. Accordingly, a fuel cell using this sheet is low in cell performance.
Fuel cell is used under various electricity-generating conditions, depending upon the application and electricity-generating method. Therefore, the gas diffusion electrode constituting each fuel cell is required to exhibit high performance under various electricity-generating conditions. A high performance is required especially under low-temperature humidity conditions.
There is a water-repellent conductive sheet obtained by producing a carbon fiber sheet, subjecting the sheet to a water-repellent treatment, and forming thereon a MPL (for example, Patent Literature 4). This water-repellent conductive sheet is too high in the hydrophobicity of whole sheet. As a result, the polymer membrane is dried especially under low-temperature humidity conditions, resulting in reduced cell performance (this phenomenon may be hereinafter referred as “dry-out”). When lower hydrophobicity is employed in order to suppress the dry-out, flooding appears, resulting in reduced cell performance.
In order to have good performance under various humidity conditions, especially under low-temperature humidity conditions, the water content inside fuel cell need be controlled. Various proposals were made for this control (for example, Patent Literatures 10 and 11).
In Patent Literature 10, there is described a method for producing a porous carbon electrode base material, which comprises laminating conductive porous base materials of different water repellencies. However, this method employs complicated steps similarly to the above-mentioned Patent Literature 3 and is low in production efficiency. Also, the knot between layers invites a higher contact resistance and accordingly low cell performance. Further, water stays easily between layers, tending to cause a reduction in performance by flooding.
In Patent Literature 11, there is described a method for producing a sheet, which comprises spraying, by air, a carbon powder, a water-repellent resin powder and a carbon fiber on a gas-permeable sheet and then spraying thereon a carbon fiber and a water-repellent resin powder, to form a two-layered structure. However, containing no binder therein, the sheet has a low strength and is inferior in handling-ability, making difficult the assembling of cell. Also, in the spraying by air, the high stiffness of carbon fiber allow the first-layer sheet containing a carbon fiber to have a low bulk density. Accordingly, part of the carbon fiber and water-repellent resin powder (both are components of the second layer) is impregnated into the first layer, giving rise to large spots, resulting in low cell performance. Furthermore, it is impossible to produce a thin sheet.